1. Field of the Invention
The present invention relates to a film thickness measuring apparatus and a film thickness measuring method which measure a film thickness of a polishing completion time point, and in particular, the present invention relates to the film thickness measuring apparatus and the film thickness measuring method which measures the film thickness of the polishing completion time point accurately without extending a strong magnetic flux against an element etc. formed on a device wafer after restraining joule thermal loss caused by an eddy current to the local minimum in a chemical mechanical polishing (CMP), etc.
2. Description of Related Art
Known is the process for forming a wiring pattern, where an oxide film is formed on a surface of a semiconductor wafer, and lithography and etching to this oxide film is performed, and a groove pattern corresponding to a wiring pattern is formed, and a conductive film which comprise Cu, etc. filling up the groove pattern, etc. is film-formed thereon, and within this conductive film, unnecessary portions other than embedded parts, such as the groove pattern and a through hole portion is removed by a chemical mechanical polishing. In formation of this wiring pattern, it is extremely important to detect surely the polishing end point when the conductive film of the unnecessary portions is removed by proper thickness to stop the process. If polishing of the conductive film is excessive, the resistance of the wiring increases, and if polishing of the conductive film is too little, it leads to an insulating failure of the wiring.
As a conventional art relevant to this, for example, the following instant monitoring method of variation of the film thickness is known. This conventional art is a method for monitoring a thickness variation of the conductive film instantaneously in the method for removing the conductive film from the substrate body (semiconductor wafer) by the chemical mechanical polishing, where a sensor including a serial or a parallel resonant circuit with an inductor and a condenser comprising a coil winded on a ferrite pot type core in order to shape so that a directivity may be brought to the electromagnetic field, is disposed adjacent to the conductive film, and a swept output comprising the frequency of 20 Hz to 40.1 MHz from a source of an excitation signal is impressed to the sensor via an impedance means for operating point setting. Thereby, when the sensor is excited, an oscillation current will flow into the coil and an alternating electromagnetic field will be generated. Subsequently, this alternating electromagnetic field induces an eddy current in the conductive film. When the eddy current is induced in the conductive film, two effects will arise. First, the conductive film operates as loss resistance, and the effect is the resistance load against the sensor circuit, and this lowers the amplitude of the resonance signal, and lowers the resonance frequency. Second, when the thickness of the conductive film decreases, the effect as if the metal rod is drawn out from the coil of the inductor will arise therein, and this will cause a variation of the inductance and the frequency shift. Thus, the thickness variation of this conductive film is made to be detected continuously by monitoring the variation of the frequency shift relevant to the sensor resonance peak resulting from the thickness variation of the conductive film (for example, refer to Japanese Patent No. 2878178 (the 2 to 7th page, FIGS. 1 to 15)).
The following eddy current sensors are known as other conventional art, for example. This conventional art comprises: a sensor coil (eddy current sensor) disposed near a base substrate on which a conductive film is formed; an AC signal source forming the eddy current in the conductive film by supplying this sensor coil with an alternating current signal with constant frequency at about 8 to 32 MHz; and a detecting circuit for measuring a reactance component and resistance component including the conductive film, and the sensor coil further comprises: an oscillation coil connected to the signal source; a sensing coil disposed to the conductive film side of this coil; and a balance coil disposed to the opposite side at the side of the conductive film of the oscillation coil, wherein the sensing coil and the balance coil are connected in a reversed phase mutually. The conventional art outputs a synthetic impedance from the resistance component and the reactance component detected in the detecting circuit, and is made to detect the variation of the film thickness of the conductive film from the variation of the impedance in a wide range as almost linear relation (For example, refer to Japanese Patent No. 3587822 (the 3rd page, FIGS. 1 to 11)).
Further, the following eddy current sensor is known as other conventional art, for example. Like conventional art of Japanese Patent No. 3587822 (the 3rd page, FIGS. 1 to 11) shown previously, in [0008], a magnetic flux which a sensor coil forms penetrates a conductive film on the substrate disposed all over the sensor coil, an eddy current is made to be produced in this conductive film because magnetic flux changes in terms of alternating, and the eddy current loss arises because the eddy current flows into the conductive film, it is saying making a reactance component of impedance of a sensor coil reduce in terms of an equivalent circuit. The [0009] thereof described that in observing the variation of the oscillated frequency of the oscillator circuit, the conductive film becomes thin gradually with progress of the polishing, and thereby, the oscillated frequency decreases down to a self-oscillation frequency of the tank circuit where the conductive film becomes completely lost by polishing, and the oscillated frequency becomes nearly constant hereafter, and therefore, by detecting this point, an end point by means of chemical machinery polishing (CMP) of the conductive film is detectable. The [0025] thereof describes that, with progress of the polishing of the conductive film, the eddy current loss changes in connection therewith, and the equivalent resistance of the sensor coil changes, and therefore, since the oscillated frequency of the oscillator circuit changes, the signal corresponding to the volume of the frequency of the detection width is displayed on the monitor by dividing this oscillating signal by the frequency divider, or subtracting with a subtractor. Thereby, the transition of the frequency locus as shown in FIG. 2 of Japanese Patent Laid-Open No. 2003-21501 is obtained (for example, refer to Japanese Patent Laid-Open No. 2003-21501).
The conventional art described above has disclosed the content which monitors the decrement of the film thickness by the eddy current in the process of the polishing in which the film thickness decreases with time frame.
Next, the well-known example with respect to a film thickness measuring apparatus in a simple static state is described. For example, a film thickness measuring apparatus in a conventional conductive film disclosed in Japanese Patent Laid-Open No. 2002-148012, comprises: an eddy current coil sensor which detects a magnetic field due to the eddy current; and a displacement sensor for measuring a displacement between the eddy current coil sensor and the measuring object films, wherein film thickness measuring apparatus is constituted so as to measure the thickness of the measuring object film, based on the inductance change amount in the eddy current sensor and the displacement amount measured with the displacement sensor.
However, the eddy current coil sensor described here is the sensor which is made adjacent to a conductive film to introduce a magnetic flux, and makes an eddy current to be induced in the conductive film, and calculates for the film thickness by computing the amount of the eddy current from the change amount of the inductance of the inductance meter. In this technology, reference is made about the change amount of inductance. That is, the inductance change amount ΔL corresponds to the eddy current loss, and if a resistivity of the conductive film is known, the film thickness can be calculated corresponding to the amount of the eddy current loss (For example, refer to Japanese Patent Laid-Open No. 2002-148012).
The film thickness measuring apparatus of the conductive film disclosed by Japanese Patent Laid-Open No. 2005-227256 discloses a technology of the film thickness measuring apparatus, the technology comprises: disposing a measuring coil near a measuring object; impressing alternating voltage to the measuring coil section; generating an eddy current in the object; and measuring a variation of the impedance value of the measuring coil arisen under the effect of the eddy current as voltage to calculate the value of the measuring object.
According to this technology, the following operation effects can be presented by using a planate coil. First, by using photo lithography and forming with a thin film on the same substrate, relative position accuracy of a coil and a displacement sensor is improved by leaps and bounds, and a jig for adjusting a relative position and work for it becomes unnecessary.
Second, a spatial resolution of a film thickness gage can be improved by using the planate coil. For example, if the frequency is raised to 5 MHz, the sensitivity of the planate threefold winded coil can be increased by 10 or more times further. Although the time frame variation of the flux linkage in a thin film becomes large because the frequency increases, and the eddy current increases, the radial-distribution state is changed. That is, the eddy current will concentrate on the zone where the diameter is small. At this time, since the film thickness measured turns into the average film thickness of the zone in which the eddy current has arisen, that the zone in which the eddy current arises is small will be to measure the average film thickness of the narrow zone, and the spatial resolution will improve as a result.
In conventional art given in Japanese Patent No. 2878178 (the 2 to 7th page, FIG. 1 to 15), the sensor is provided with the serial or parallel resonant circuit with an inductor and a condenser comprising the coil winded on a ferrite pot type core for bringing the directivity to the electromagnetic field. A swept output having the frequency of 20 Hz to 40.1 MHz in the polishing early phase is impressed to the sensor, and by an alternating electromagnetic field with the directivity generated from the coil, a magnetic leakage flux penetrating the conductive film is made to be arisen, and a large eddy current corresponding to the film thickness of this conductive film is made to be induced from the polishing early phase. In order to induce a large eddy current corresponding to the film thickness of the conductive film, it is required to form a large magnetic flux so as to penetrate the conductive film, that is a large alternating electromagnetic field, and the monitor of the thickness variation of the conductive film is carried out from the polishing early phase to the polishing ending phase, using the eddy current induced in the conductive film. Accordingly, it is required during the monitor of the thickness change to make the magnetic flux penetrate toward the thickness direction of the conductive film. In a figure of Japanese Patent No. 2878178 (the 2nd to 7th page, FIGS. 1 to 15), this is also clear from the lines of magnetic flux penetrating this conductive film into the portion of all the conductive films being indicated.
It is common in the surface of the wafer in the polishing early phase that a pure Cu film (conductive film) is in the top layer. In order to make all these pure Cu films induce the eddy current, very large magnetic leakage flux is required. However, although the magnetic leakage flux induces the eddy current, which becomes Joule's heat to be consumed in the form of the eddy current loss after all. As for this joule thermal loss, for the pure Cu film on the top surface layer, the generation of heat is comparatively small, since the volume resistivity thereof is small. However, in the inner part wired thereon already, since a wiring cross section is small and the volume resistivity is small, a large eddy current is induced by penetrating magnetic flux, and as a result, large joule thermal loss will be induced locally. This problem sometimes develops into the melting and breaking of the wiring in a part. The so-called state of the induction heating will be caused, and a phenomenon in which the inside thereof is particularly filled with heat will be introduced. In particular, in Cu wiring, etc. it is concerned that heating of Cu will diffuse Cu in barrier films, such as Ta and in the case, a barrier film will be broken through and Cu will be diffused.
In the case of wiring is given to the surface part in many layers of the wafer, there are not only concerns about the surface Cu film, but also the concerns that a wiring part of the inner part of which processing has already completed is heated locally to make the effects diffuse around, and a dopant which forms a p-type and a n-type in the semiconductor substrate is diffused further to change also the characteristics of elements in the substrate. In the case of excessive eddy current flows into fine wiring, even when heat is not generated, an electromigration is caused and it may be made to result in an open circuit.
Further, for example, in the case of performing process by changing a polishing condition, at the time point when the amount of residual films reached at the predetermined value around the polishing end time point, it is difficult to discern whether the amount of residual films is the predetermined amount. It is because, although it is possible to guess based on the changed part from the early phase film thickness, if the early phase film thickness varies, the estimate of the predetermined amount of residual films will vary. With regard to the decision of the portion around the polishing end time point, if the gap between the sensor and the conductive film changes with the vibration of the polishing minutely, stray capacitance of the overall sensor circuitry changes and the overall resonance frequency shifts. Accordingly, even if the setting which discriminates the polishing end point is performed with setting up the threshold value corresponding to the resonance frequency set at a certain value, if the resonance frequency shifts on the whole, the decision of the polishing end time point based on the setting of the threshold value becomes difficult. Thus, in the resonance frequency which increases or decreases monotonously and continuously in a conventional art, even if the threshold value is set as a certain value, because a gap between a sensor and a conductive film changes minutely or a certain dielectrics intervene between them, it existed frequently that the waveform itself overall did parallel displacement up and down, and as a result, it existed frequently that the threshold value which was set up in advance did not make a meaning.
Also in conventional art given in Japanese Patent No. 3587822 (the 3rd page, FIGS. 1 to 11) using an eddy current sensor, it is almost the same as that of conventional art given in the above-mentioned Japanese Patent No. 2878178 (the 2nd to 7th page, FIGS. 1 to 15) that a monitor of thickness change of a conductive film, is performed based on variation of the eddy current from the polishing early phase to the polishing ending phase.
By the above-mentioned conventional art which monitors the film thickness of the conductive film from the polishing early phase to the polishing ending phase using the eddy current, it is necessary to make sufficiently strong magnetic flux to the extent of infiltrating into the film to cause the eddy current therein, and the shape of the inductor has became three-dimensioned, in order to give a directivity to the magnetic flux. Accordingly, when building a sensor into a polishing apparatus etc., there are generally the following problems. A current fed through a coil becomes large, power dissipation increases and power supply equipment also becomes large-sized. A magnetic flux leaks on the circumference and it is easy to generate a noise. A process step of coiling lead wire around a coiled form etc. is needed, and the cost becomes high.
In conventional art to comprise the eddy current sensor according to Japanese Patent Laid-Open No. 2003-21501, about hardware of the sensor portion used by this conventional art, first the sensor coil is configured on condition of penetrating the conductive film. Therefore, by hardware which generates only a magnetic field of the extent which does not penetrate the conductive film, the eddy current cannot be formed and the object cannot be attained. As the conductive film thickness decrease by the polishing, zones in which the eddy current is formed decrease monotonously, and a behavior in which an oscillated frequency decreases monotonously therefore is indicated there, and a time point when the oscillated frequency becomes nearly constant is intended to be deemed as the end point to be detected. In the algorithm of software used by this conventional art, as for a variation of the oscillated frequency, the variation where the frequency decrease down to a nearly constant level is considered as the variation of the oscillated frequency, and in the case of this oscillated frequency has changed as having an inflection point, for example, the algorithm cannot possibly detect the end point. There considered is the state which the magnetic flux penetrates the conductive film from the early phase of the polishing as shown in FIG. 2 of Japanese Patent Laid-Open No. 2003-21501, and the eddy current is always generating. Here, the eddy current sensor generates the eddy current positively from beginning to end, and the method of recomputing the thickness change from the eddy current variation is generally made to be the eddy current sensor.
An eddy current sensor given by Japanese Patent Laid-Open No. 2002-148012 is based on a technology having the premise of introducing the magnetic field into the conductive film theoretically. Here, the way how to take out the eddy current describes that a inductance change amount is computed and calculated using an inductance meter, and as phenomenalism, the principle has no change in being a general principle which makes magnetic field infiltrate an inner part of the conductive film, and detects the amount of eddy currents corresponding to the conductive film.
As a result, in the case of the film thickness of the conductive film formed over device elements is measured, the magnetic field will be given even to the internal device element at the same time the magnetic field is made to infiltrate the conductive film. At this time, in the case of the conductive film is thick, the energy of the magnetic field is absorbed as joule thermal loss caused by the eddy current formation in the conductive film. In the case of the conductive film is extremely thin, the given magnetic field is not interrupted by the surface of the conductive film, and infiltrates into the device element as it is. Consequently, an excessive eddy current is formed in the wiring section inside the device, problems of electromigration, etc. inside wiring, are induced as a result. In the case of measuring the film thickness by the variation of the amount of the eddy current, since the eddy current itself induced becomes weak gradually when the film thickness is extremely thin, measuring sensitivity will be reduced extremely as a result.
In a technology given by Japanese Patent Laid-Open No. 2005-227256, as a film thickness measurement principle, using change amount ΔL of an inductance component of planate coil 11 for measurement, and distance between planate coil 11 for measurement and conductive film 51, based on a data base of these correlation relationships calculated in advance, a film thickness of the part concerned on conducting film 51 is computed. Therefore, basically corresponding to the amount of the eddy current, the film thickness is determined.
Here, there not seen is the description that a variation of an eddy current has an inflection point, and the film thickness measurement is performed using the inflection point. Further, with regard to the conductive film, two aspects where the magnetic flux penetrates and does not penetrate based on the skin effect, are given, and utilizing the variation of the eddy current state in the two aspects, the film thickness is calculated is not described.
This technology has a forward direction coil and also an inversely winded reverse coil. A field which expands outside by the forward direction coil is made to be negated with this reverse coil. That is, since the magnetic flux to diverge is negated with the reverse coil and the magnetic flux concentrates on the middle position of the forward direction coil, the resolution can be improved. Here, even if a planate inductor is used, the planate inductor is made to have the role negating the magnetic flux which will not penetrate into the conductor film by selecting out only magnetic flux which penetrates the conductor film around the coil center section purely.
Such a method is not intending that the magnetic flux located on the circumference rather than the magnetic flux of the center section of the coil is used, in order to make magnetic flux not infiltrate the conductive film. The magnetic flux is made to penetrate positively at around the center section of the coil, and the penetrating magnetic flux is guided locally. It is premised on making the magnetic field infiltrate positively in the part in the conductive film.
From the above thing, the technology of Japanese Patent Laid-Open No. 2005-227256 uses the magnetic flux of the center section of the coil, makes the magnetic field infiltrate positively in the conductor film, removes the flux in the circumference of the coil, and makes the magnetic flux infiltrate in the conductor film only at the directory subjacent portion. The technology makes it important to make the magnetic field infiltrate the conductor basically, and is constituted for the purpose of eliminating the magnetic flux portion which does not infiltrate the conductor of the outer peripheral part of the coil. Japanese Patent Laid-Open No. 2005-227256 where the planate inductor is used makes it the base to remove the portion which the magnetic flux does not infiltrate, with making it a prerequisite to make the magnetic flux always penetrate.
However, as mentioned previously, even when the magnetic field is made to penetrate the conductor even locally, since a lot of magnetic fields become the eddy current to be consumed within the conductor, the heat is abruptly generated by the impressed magnetic field in a wiring part which exists particularly under the surface of the conductive film, and the disconnecting of wirings by electromigration, depending on the case, is also considered.
Here, using the planate inductor does not lead immediately to acquiring the skin effect as it is. Although it is also important to form a dispersed magnetic field using the planate inductor in order to acquire the skin effect, the skin effect is influenced by a frequency of an alternating magnetic field, and conductivity and permeability of the conductive film to be used, and is also dependent on the distance of the coil and the conductive film.
Therefore, by means of the skin effect, in the case of forming the state where magnetic flux penetrates relatively, and the state where magnetic flux does not penetrate relatively, setting up must be performed with factoring-in not only the shape of the inductor but the frequency to be used and the conductivity and permeability of the conductive film, etc.
It is important to form the state where the magnetic field penetrates the conductive film relatively, and the state where the magnetic field does not penetrate the conductive film relatively by rationalizing such setting. It becomes possible to calculate various values (film thickness, conductivity, permeability) of the conductive film based on the transition process of the polishing by utilizing the two changes of state. Although the skin effect expresses a phenomenon in which electromagnetic waves do not infiltrate even an inner part of the conductor, the skin effect as a phrase expression is used in the meaning corresponding the critical state of the state where the magnetic field penetrates the conductor film, and the state where the magnetic field does not penetrate the conductor film.
As a method to measure film thickness of a conductive film, besides such a well-known example, there exists also a method in which resistivity of the conductive film is measured by four probes, and from the resistivity thereof the film thickness is measured. However, since measuring is performed with direct contact to the conductive film, the conductive film surface may be damaged, and because of a state of the metallic film of the probe tip and a contact resistance of the probe tip and the wafer surface, etc., the measured film thickness value may deviate substantially.
Then, in order to improve the above conventional art, the following subject arises. That is, without extending a strong magnetic flux even on a microscopic wiring formed in the film, consequently by suppressing a generation of an eddy current induced by electromagnetic induction, the joule thermal loss caused by the eddy current needs to be restrained to the local minimum. Further, it is necessary to eliminate disconnecting wirings in the device element by the electromigration caused by the eddy current formation. The situation needs to be eliminated that setting of the threshold value changes substantially and detection become difficult to be performed, because the amount of the eddy current induced shifts on the whole by the variation of the gap between the sensor and the conductive film and dielectric matter such as slurry intervening therebetween.
Even if the magnetic field is a microscopic magnetic field of the extent which does not penetrate the device wafer, it is necessary to enable it to detect with accuracy sufficient enough. It is necessary to enable it to measure without contacting the film and without damaging the element surface. It is necessary to enable it to measure the desired amount of thin film with sufficient sensibility.
Therefore, the polishing end time point is estimated and detected with sufficient accuracy, and the amount of residual films to be removed and a polishing rate, etc. is estimated instantaneously with sufficient accuracy, and in order to evaluate accurately whether a predetermined conductive film is removed properly and has become desired film thickness, the technical problem which should be solved arises, and the object of the present invention is to solve this problem.